Mercury blast splashed Earth

These panels illustrate how a giant impact could have depleted Mercury’s silicate crust and mantle (blue) while leaving behind an oversize iron core (red). Left: An object half Mercury’s present mass collides obliquely with a 2.25-Mercury-mass protoplanet. This view shows the model 2 minutes after the impact. Center: Within 8 minutes of the collision, both worlds are shattered. Right: This wider view shows the scene 3 hours later. Much of the rocky material has been removed and little of it will return.

Jonathan Horner, University of Bern

April 5, 2006New computer simulations of a giant impact on the young planet Mercury shed light on the planet's odd makeup. Moreover, say scientists, debris from this cosmic collision could have made its way to Earth and Venus.

"Mercury is an unusually dense planet, which suggests that it contains farmore metal than would be expected for a planet of its size," says Jonathan Horner at the University of Bern, Switzerland. While Earth ranks as the solar system's densest planet, Mercury comes in a close second despite containing just 5.5 percent of Earth's mass. Results from the Mariner 10 spacecraft, which flew past the planet 3 times in the mid-1970s, explained why: Mercury possesses an iron core nearly as large as the planet itself.

Scientists had long suspected this supersize core arose early in Mercury's history through a catastrophic collision that blasted away most of the planet's rocky crust and mantle. But, says Horner, "Until these simulations, we were not sure why so little of the planet's outer layers were reaccreted following the impact."

Horner and his colleagues Augustin Anic, James Whitby, and Willy Benz simulated collisions between a proto-Mercury 2.25 times the planet's current mass and an impactor half of Mercury's present mass moving at 62,600 mph (100,000 km/h). The result was a dense, metal-rich body and a swath of escaping debris. "The simulations … are very promising," Horner tells Astronomy. "They seem to create an object very much like the Mercury we see, which is obviously a good sign for the theory." He presented the study at the Royal Astronomical Society's National Astronomy Meeting April 4.

To follow what happened to the ejected matter, the team fed information about the debris trajectories into a second set of simulations to see how long particles drifted in space before the newly formed Mercury swept them up. The simulation tracked particles until they either landed on a planet, fell into the Sun, or traveled beyond Jupiter.

The scientists found that half of the ejecta could take as long as 4 million years to fall back to the planet. That's sufficient time for non-gravitational forces not included in the model — such as radiation pressure from sunlight or the Poynting-Robertson effect  to alter particle orbits. Most debris in the scientists' impact scenario would never return to Mercury.

Interestingly, some of the debris could have reached Venus and even Earth. "It really demonstrates how material spreads out after such an event," Horner says. He thinks Earth may contain as much as 17 million billion tons (16 million billion metric tons) of proto-Mercury.

NASA's Messenger spacecraft, now en route, will be the first spacecraft to visit Mercury since Mariner 10. Messenger is scheduled to fly within 124 miles (200 kilometers) of Mercury's surface in January and October 2008 and September 2009. The probe will enter into Mercury orbit in March 2011.

View the movie to see a simulated Mercury impact from the University of Bern group.